The present invention relates generally to a giant magnetoresistive sensor for use in a magnetic read head. In particular, the present invention relates to a giant magnetoresistive read sensor having improved thermal and magnetic stability.
Giant magnetoresistive (GMR) read sensors are used in magnetic data storage systems to detect magnetically-encoded information stored on a magnetic data storage medium such as a magnetic disc. A time-dependent magnetic field from a magnetic medium directly modulates the resistivity of the GMR read sensor. A change in resistance of the GMR read sensor can be detected by passing a sense current through the GMR read sensor and measuring the voltage across the GMR read sensor. The resulting signal can be used to recover the encoded information from the magnetic medium.
A typical GMR read sensor configuration is the GMR spin valve, in which the GMR read sensor is a multi-layered structure formed of a nonmagnetic spacer layer positioned between a ferromagnetic pinned layer and a ferromagnetic free layer. The magnetization of the pinned layer is fixed in a predetermined direction, typically normal to an air bearing surface of the GMR read sensor, while the magnetization of the free layer rotates freely in response to an external magnetic field. The resistance of the GMR read sensor varies as a function of an angle formed between the magnetization direction of the free layer and the magnetization direction of the pinned layer. This multi-layered spin valve configuration allows for a more pronounced magnetoresistive effect, i.e. greater sensitivity and higher total change in resistance, than is possible with anisotropic magnetoresistive (AMR) read sensors, which generally consist of a single ferromagnetic layer.
A pinning layer is typically exchange coupled to the pinned layer to fix the magnetization of the pinned layer in a predetermined direction. The pinning layer is typically formed of an antiferromagnetic material. In antiferromagnetic materials, the magnetic moments of adjacent atoms point in opposite directions and, thus, there is no net magnetic moment in the material.
A seed layer is typically used to promote the texture and enhance the grain growth of the free layer consequently grown on top of it. The seed layer material is chosen such that its atomic structure, or arrangement, corresponds with the preferred crystallographic direction of the magnetization of the free layer material.
Principal concerns in the performance of GMR read sensors are the thermal and magnetic stability of the GMR read sensor. When the pinning layer is exchange coupled to the pinned layer during manufacturing, the GMR read sensor must not experience diffusion between thin layers in the GMR read sensor. After the pinning layer is exchange coupled to the pinned layer to fix the magnetization of the pinned layer in a predetermined direction, a sufficient exchange coupling field, or pinning field, is required to keep the magnetization of the pinned layer, unchanged during the operation of the GMR read sensor.
Key determinants of the thermal and magnetic stability of the GMR read sensor are the materials used as the pinning layer and as the seed layer in the GMR read sensor. The annealing temperature of the pinning layer material is the temperature at which the pinning and pinned layers are exchange coupled during manufacturing. It is desirable for the pinning layer material to have a low annealing temperature to control magnetic behavior and prevent diffusion between thin layers in the GMR spin valve. The blocking temperature of the pinning layer material is the temperature at which the exchange coupling disappears. It is desirable for the pinning layer material to have a high blocking temperature to prevent the magnetization of the pinned layer from changing during the operation of the GMR read sensor. Also, the seed layer material affects the strength of the exchange coupling field, or pinning strength. It is desirable for the seed layer material/pinning layer material combination to have a high pinning strength to keep the magnetization of the pinned layer unchanged.
Accordingly, there is a need for a pinning layer material with a high blocking temperature and a low annealing temperature, and for a seed layer material that, when used with the pinning layer material, produces a high pinning strength.
The present invention is a giant magnetoresistive stack for use in a magnetic read head. The giant magnetoresistive stack includes a NiFeCr seed layer, a ferromagnetic free layer, a nonmagnetic spacer layer, a ferromagnetic pinned layer, and a PtMnX pinning layer, where X is either Cr or Pd. The free layer has a rotatable magnetic moment and is positioned adjacent to the NiFeCr seed layer. The pinned layer has a fixed magnetic moment and is positioned adjacent to the PtMnX pinning layer. The spacer layer is positioned between the free layer and the pinned layer. In a first preferred embodiment, the free layer is a NiFe/CoFe bilayer, the spacer layer is formed of copper, and the pinned layer is formed of CoFe. In a second preferred embodiment, the free layer is a NiFe/CoFe bilayer, the spacer layer is formed of copper, and the pinned layer is a CoFe/Ru/CoFe synthetic antiferromagnet.